1. Field of the Invention
This invention relates to a thick grain-oriented electrical steel sheet exhibiting excellent magnetic properties and suitable for use as the material for the core of a transformer or the like.
2. Description of the Prior Art
Since grain-oriented electrical steel sheet is used mainly as a core material for transformers and other electrical equipment, it is required to exhibit excellent magnetic properties, most notably excellent magnetization property and core loss property. Magnetization property is generally expressed as the flux density B8 value at a magnetic field of 800 A/m, and core loss property is expressed as the W17/50 core loss value at a frequency of 50 Hz and a magnetization to 1.7 Tesla.
The main factor governing core loss property is flux density. Generally speaking, the higher the flux density, the better is the core loss property. Notwithstanding, increasing the flux density causes the secondary recrystallization grain size to be enlarged simultaneously and, to the extent that it does, has a degrading effect on the core loss property. In contrast, magnetic domain control enables an improvement in core loss property irrespective of the secondary recrystallization grain diameter.
Grain-oriented electrical steel sheet is produced with use of secondary recrystallization phenomenon in the final annealing step so as to develop a Goss texture wherein the grains have their (110) axes aligned with the sheet surface and their <001> axes aligned with the rolling direction. For obtaining good magnetic properties, the easily magnetizable <001> axis has to have a high degree of alignment with the rolling direction.
JP-B-40-15644 and JP-B-51-13469 teach typical methods for producing a high flux density grain-oriented electrical steel sheet. JP-B-40-15644 describes a method using MnS and AlN as the main inhibitors and JP-B-51-13469 describes a method of using MnS, MnSe, Sb and the like as the main inhibitors. Appropriate control of the size, morphology and distribution of the precipitates functioning as inhibitors is therefore an indispensable requirement in the currently available technology.
On the other hand, owing to the desire of transformer manufacturers to increase the energy efficiency and lower the cost of their products, the laminated core sector has experienced increasing need for thick grain oriented electrical steel sheet enabling a reduction in the number of laminations. Moreover, the large rotating machine sector has also long showed an interest in using grain-oriented electrical steel sheet. Here again the need is particularly high for thick grain-oriented electrical steel sheet that allows the number of laminations to be reduced.
Since increasing sheet thickness generally leads to degradation of core loss property, a strong need has arisen for the development of a thick grain-oriented electrical steel sheet with excellent magnetic properties.